1. Field of Invention
This invention relates to telescopic and other optical sighting systems for use on guns and other projectile delivering systems of all types but will herein it will be described as primarily applied to telescopic sights on typical rifles. More particularly, this invention relates most naturally to a telescopic gunsight equipped with a transparent aiming reticle and a method for using that reticle on a gun but it is certainly not constrained to that specific application.
2. Description of Prior Art
It is well known that the line of sight between a shooter's eye and a target is a straight line, whether using “iron” sights or a telescopic sight, while the trajectory of the projectile is never a straight line (when passing though a gravitational field, the trajectory follows a ballistic parabola), which becomes of particular importance for projectile flights covering long distances. Accordingly, in order to achieve sufficiently accurate shot placement, it is essential either to “sight in” the rifle (or other projectile delivering system: hereafter “gun”) to produce the projectile (hereafter, bullet) to the desired aiming point at a specific fixed target distance or to know both the distance from the gun to the target and the trajectory characteristics of the bullet used. Trajectory characteristics for commercial bullets are related to initial launch velocity and are known or are easily obtained from either the manufacturer or from trajectory charts such as INGALLS' tables.
Telescopic gunsights, often referred to as “scopes,” generally contain a transparent flat disk reticle positioned in a plane perpendicular to the line of sight through the scope. The reticle conventionally has a single vertical crosshair (or hairline) and a single horizontal crosshair (or hairline), which intersects the vertical hairline near the visual center of the reticle and the scope. The point of intersection of these crosshairs constitutes the primary sighting point for the scope, representing site of bullet impact at a chosen (zero) distance.
In modern scopes, the gunsight is most commonly moveable in vertical and horizontal directions by way of calibrated adjustment screws located on the scope exterior (internal adjustments); in some older and a few newer and new scopes, the gunsight is adjusted by devices within the scope attachment system (external adjustments). Method of adjustment has no significant influence upon reticle design or use.
By firing one or more shots and making compensatory adjustments of the relative position of the reticle center point, the shooting system, which is comprised of rifle, bullet type and velocity, scope and shooter is “zeroed in” so that aiming position of the reticle crossed hairlines or reticle center point coincides with point of bullet impact on the target.
In certain scope sighting systems, the reticle has a series of evenly-spaced secondary horizontal hairlines that intersect the vertical hairline below the center horizontal hairline. In those systems, the respective points of intersection of the secondary hairlines with the vertical hairline are typically used to estimate bullet impact points at distances progressively greater than that at which the rifle was “zeroed in” with the main (center) horizontal crosshair. However, in order to utilize these secondary horizontal crosshairs with accurate and predictable results, the shooter must know distance from gun to target with a significant degree of precision.
Various types of range finder systems have been disclosed for telescopic gunsights. For example, U.S. Pat. No. 1,190,121 to Critchett discloses a reticle having a series of target-spanning rulings disposed above a baseline, the rulings corresponding to associated shooting distances. In use, the shooter ascertains which ruling above the baseline makes the most closely embracing fit on the target, thereby determining the shooting distance (target range). A separate crosshair aiming point is included in the reticle for use in association with each chosen ruling above the baseline.
The principle of the Critchett target-spanning rulings is that certain targets are of known, or at least estimable size. For instance, it is a fairly accurate estimate that for mature deer or antelope, the distance between the top of the back at the shoulders and the bottom of the chest cavity is about 18 inches. The target-spanning rulings are spaced apart such as to span a known target size at a known range. This manner of distance measurement is consistent with conventional trigonometric considerations wherein the triangle defined by the height of the target and the viewing angle through the telescope's optical system can be considered a right triangle, which accordingly establishes the length of the base line distance to the distal side of the triangle, namely the distance to the target.
U.S. Pat. No. 3,392,450 to Herter et. al. discloses a reticle having a series of target-spanning circles of different diameters which correspond to associated shooting distances. Employing the same basic distance-measuring concept as Critchett, the shooter employs for aiming purposes, that crosshair which corresponds to the selected circle.
U.S. Pat. No. 3,190,003 to O'Brien concerns a range-finding reticle for a telescopic gunsight having single centered vertical and horizontal hairlines. The portion of the vertical hairline below the horizontal centerline is provided with widened bar regions extending various lengths below the centerline. Each bar subtends a target of known size. By finding which widened region corresponds to the height of the target, the shooting distance is estimated.
U.S. Pat. No. 3,431,652 to Leatherwood discloses a telescopic gunsight wherein the distance to the target is determined by movement of upper and lower horizontal hairlines along a fixed vertical hairline in a manner so as to bracket the target. Once bracketed, the intersection of the lower horizontal hairline with the vertical hairline serves as the crosshair aiming point. In this aiming process, the alignment of the scope changes with respect to the gun barrel, whereby the allowance for distance is achieved when the centered crosshair is sighted directly on the target.
U.S. Pat. No. 3,492,733 to Leatherwood discloses a distance measuring system for a variable power telescopic sight that is pivotally moveable in a vertical plane with respect to the gun barrel upon which it is mounted. Cams within the scope and rotatable by external means achieve vertical movement of the scope so that horizontal framing hairlines will fit the target. A specialized cam must be installed into the scope for each particular type of ammunition employed.
U.S. Pat. No. 3,948,587 to Rubbert concerns a variable power telescopic sight having a reticle provided with a vertical hairline, a center horizontal hairline and three horizontal framing lines disposed below the center horizontal hairline. Aiming is achieved by positioning either the center crosshair or lower crosshairs on the target, as dictated by the observed fit of the target within the framing lines.
U.S. Pat. No. 4,403,421 to Shepherd discloses a telescopic gunsight having spaced apart primary and secondary reticles which are moveable relative to each other. The secondary reticle is also moveable vertically and horizontally within the plane of the reticle. The moveable two reticle system facilitates adjustments for windage and elevation. Distance to the target is ascertained by framing indicia on the secondary reticle.
The telescopic sights disclosed in the aforementioned prior art patents are often of limited usefulness insofar as they do not address many of the several factors that need to be considered in the accurate aiming of a rifle under field conditions. Such factors include:
a) distance to target
b) drop of bullet caused by force of gravity
c) hold-over or hold-under aiming points
d) wind drift correction
e) correction for phenomenon associated with gyroscopic forces on a gyroscopically stabilized bullet (sometimes referred to as)
1) Yaw of Repose effects (vertical displacements)
2) Magnus effects (horizontal displacements)
These latter result from the effect of cross-wind or shooting either up-hill or down-hill.
Older reticle systems often require that the shooter look away from the target in order to make compensating adjustments and almost always require complicated mental or physical manipulations. Some of these designs may render the scopes difficult or slow to use, and some require moveable mounting on the rifle, a situation which typically subjects the scope to inaccuracy after repeated use or abuse in rugged field conditions. Moreover, correct use of any of these systems always requires the shooter to manage extraordinary mental work in what can already be a stressful situation. It is proven that such additional stress is associated with decreased performance potential.